Seven NATO countries completed naval exercises in the Black Sea on Thursday, officials said, in a signal of the alliance's resolve to support east European members who have been unnerved by Russia's behavior in the Ukraine crisis.

Though scheduled before the crisis erupted, the war games took on added significance because of Russia's annexation of Ukraine's Black Sea peninsula of Crimea in March, and the subsequent outbreak of a pro-Russian separatist rebellion in eastern Ukraine.

Led by the Bulgarian navy, the 10-day drills aimed to improve tactical coordination of allied units “in a multi-threat environment,” NATO has said. Vessels took part from Bulgaria, Romania, Turkey, Greece, Italy, Britain and the United States.